A system for detecting vehicles and suitable for use in conjunction with other equipment for traffic monitoring and control. The system includes one or more sensors capable of measuring and indicating magnetic field intensity and a central electronic unit which interprets the output signals produced by the sensors. The sensors preferably comprise flux gate magnetometers. The system can be operated in either of two modes; i.e. a "between lanes" mode (I) in which each sensor is mounted adjacent to a lane being monitored and an "in lane" mode (II) in which each sensor is mounted in a lane being monitored. In mode I the central unit produces a detect output signal in response to a magnetic field intensity reduction, as would be caused by a vehicle passing in a monitored lane, which exceeds a first threshold level. In mode II, the detect output signal is produced in response to a sensed magnetic field intensity increase, as would be caused by a vehicle passing in a monitored lane, which exceeds a second threshold level. Both the first and second threshold levels are established as some percentage of the ambient magnetic field intensity. A bias signal, developed as a function of the sensed ambient field intensity, is fed back to the sensors to null their output under quiescent conditions, thus enabling the system to automatically adapt to different ambient field conditions. Time delay means are incorporated in the feed back loop producing the bias signal to prevent the system from adapting to short term magnetic field changes.

A system useful for indicating the entry of a vehicle onto a specified area of the earth's surface. The system includes a magnetic field sensor which yields an output signal indicative of the magnetic field intensity thereat. The sensor is mounted adjacent to but outside of a volume being monitored which constitutes the projection of the specified area in the direction of the magnetic field thereat. When the magnetically permeable mass of a vehicle enters the volume being monitored, it increases magnetic field intensity therein but reduces the field intensity outside of this volume. The reduction in field intensity is recognized by the sensor which then energizes an indicating device, which in the case of a service station installation, for example, can be a remote bell. The sensor preferably comprises a flux gate magnetometer mounted within a thin substantially rigid housing dimensioned to fit within a slot formed by saw cutting a roadway surface.

An underground vehicle sensing system extends below a vehicle travel surface to detect the presence of a vehicle. A conduit extends substantially horizontally underground from an access hole placed at the side of the vehicle travel surface. A plurality of sensors are spaced apart in the conduit to detect localized changes in the magnetic field. A plurality of sections house the probes and wiring to orient and position the probes. Sections detachably clip together to form a lightweight support structure and may be added one by one through the access hole. Lightweight extension members are utilized in some configurations between the support sections for positioning and supporting the probes.

An induction loop vehicle detection system and method of making and installing such a system are disclosed wherein the conduit sections containing the induction loop are preformed and prewired integrally with all interconnects by using removable expansion joints to assemble sections of prewired and sealed conduit in any desired configuration. The expansion joints can be removed after assembly to enable the induction loop to be folded down for shipment. The expansion joints also enable longitudinal displacement of the induction loop conduit due to thermal differential expansion during installation and during extreme changes in temperature over the lifetime of the system, thereby preserving the integrity of the roadway and fault free operation of the induction loop. The wires forming the induction loop are completely sealed within the conduit with an asphalt rubber material thereby precluding any moisture penetration. The induction loop vehicle detector is installed by a method of dry routing of pavement which eliminates labor intensive slurry clean-up. The conduit is sealed in the routed slot in a roadway with a pressure withstanding asphalt rubber material which tenaciously adheres to the side surfaces of the routed slot.

The present invention is directed to a method and apparatus for measuring a magnetic field and determining whether an anomaly is present based on the magnetic field measurement and a measured ambient magnetic field. The present invention is also directed to a method and apparatus for measuring the intensity of the magnetic field by implementing operations to essentially eliminate error components that arise due to factors including offsets and gains affecting the intensity signal processing.